Electrode for solid-state batteries and solid-state battery

ABSTRACT

An electrode for solid-state batteries, comprising a PTC resistor layer, and a solid-state battery comprising the electrode. The electrode may be an electrode for solid-state batteries, wherein the electrode comprises an electrode active material layer, a current collector and a PTC resistor layer disposed between the electrode active material layer and the current collector; wherein the PTC resistor layer contains an electroconductive material, an insulating inorganic substance and a polymer; and wherein a porosity of the PTC resistor layer is from 5% to 13%.

TECHNICAL FIELD

The disclosure relates to an electrode for solid-state batteries and a solid-state battery comprising the electrode.

BACKGROUND

In a battery used as an in-vehicle power source or as a power source for notebook PCs and portable devices, the temperature of the whole battery may increase due to an internal short circuit or overcharging and may have adverse effects on the battery itself or on a device using the battery.

As a measure to prevent the adverse effects, a technique of using an electrode has been attempted, the electrode comprising a positive temperature coefficient (PTC) resistor layer which has electron conductivity at room temperature and which shows an increase in electronic resistance value with an increase in temperature.

Patent Literature 1 discloses an all-solid-state state battery comprising a laminate of a cathode active material layer, a solid electrolyte layer, and an anode active material layer in this order, and a restraining member that applies a restraining pressure to the laminate in a laminated direction, wherein a PTC layer containing a conductive material, an insulating inorganic substance and a polymer, is disposed at least at one of a position between the cathode active material layer and a cathode current collecting layer for collecting electrons of the cathode active material layer, and a position between the anode active material layer and an anode current collecting layer for collecting electrons of the anode active material layer, and the content of the insulating inorganic substance in the PTC layer is 50 volume % or more.

Patent Literature 2 discloses an all-solid-state battery comprising: a cathode layer comprising a cathode active material layer and a cathode current collector; an anode layer comprising an anode active material layer and an anode current collector; and a solid electrolyte layer disposed between the cathode active material layer and the anode active material layer, wherein the all-solid-state battery further comprises a PTC film between the cathode current collector and the cathode active material layer and/or between the anode current collector and the anode active material layer, and the PTC film contains a conductive material and a resin.

-   Patent Literature 1: Japanese Patent Application Laid-Open (JP-A)     No. 2018-014286 -   Patent Literature 2: JP-A No. 2017-130283

However, the electrode as disclosed in Patent Literature 1, the electrode comprising the PTC resistor layer containing the insulating inorganic substance, has a problem in that electronic resistance at the interface between the PTC resistor layer and the electrode active material layer in a room temperature condition (15° C. to 30° C.) is large. The electrode as disclosed in Patent Literature 2, the electrode comprising the PTC resistor layer not containing the insulating inorganic substance, has a problem in that electronic resistance is decreased in a high temperature condition due to the effects of confining pressure.

SUMMARY

The disclosed embodiments were achieved in light of the above circumstance. An object of the disclosed embodiments is to provide an electrode for solid-state batteries, comprising at least a PTC resistor layer in which electronic resistance in a room temperature condition is low. Another object of the disclosed embodiments is to provide a solid-state battery comprising the electrode.

In a first embodiment, there is provided an electrode for solid-state batteries, wherein the electrode comprises an electrode active material layer, a current collector and a PTC resistor layer disposed between the electrode active material layer and the current collector; wherein the PTC resistor layer contains an electroconductive material, an insulating inorganic substance and a polymer; and wherein a porosity of the PTC resistor layer is from 5% to 13%.

The insulating inorganic substance may be a metal oxide.

The electroconductive material may be carbon black.

In another embodiment, there is provided a solid-state battery comprising a cathode, an anode and an electrolyte layer disposed between the cathode and the anode, wherein at least one of the cathode and the anode is the above-mentioned electrode for solid-state batteries.

For the electrode for solid-state batteries according to the disclosed embodiments, the porosity of the PTC resistor layer is in the specific value range, whereby more electron-conductive paths than ever before are contained inside the PTC resistor layer, and excellent electron conductivity inside the PTC resistor layer is obtained. As a result, when the electrode is used in a solid-state battery, an increase in the electronic resistance inside the PTC resistor layer can be suppressed, and a decrease in the performance of the solid-state battery can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a view showing an example of the layer structure of the electrode for solid-state batteries according to the disclosed embodiments, and it is also a schematic cross sectional view of the electrode along the laminating direction;

FIG. 2 is a view showing an example of the layer structure of the solid-state battery of the disclosed embodiments, and it is also a schematic cross sectional view of the solid-state battery along the laminating direction;

FIG. 3 is a schematic view of a circuit for electronic resistance measurement, which includes an evaluation sample; and

FIG. 4 is a view showing a relationship between the electronic resistance of an evaluation sample of an electrode and the resistance of a solid-state battery comprising the electrode.

DETAILED DESCRIPTION 1. Electrode for Solid-State Batteries

The electrode for solid-state batteries according to the disclosed embodiments is an electrode for solid-state batteries, wherein the electrode comprises an electrode active material layer, a current collector and a PTC resistor layer disposed between the electrode active material layer and the current collector; wherein the PTC resistor layer contains an electroconductive material, an insulating inorganic substance and a polymer; and wherein a porosity of the PTC resistor layer is from 5% to 13%.

It is known that if a layer containing an electroconductive material and a polymer is disposed between the electrode active material layer and the current collector, the layer shows a PTC resistor function (a rapid increase in electronic resistance) when the temperature of the layer exceeds the melting point of the polymer by heating. The PTC resistor function is exerted when the particles of the electroconductive material, which are in contact with each other, are separated by thermal expansion of the polymer and result in blocking of electron conduction. In the disclosed embodiments, the layer showing such a PTC resistor function is referred to as “PTC resistor layer”.

In the solid-state battery comprising the PTC resistor layer, when the temperature of the solid-state battery is increased due to overcharging or a short circuit, electron conduction between the electrode active material layer and the current collector is blocked, and an electrochemical reaction is arrested. Accordingly, a further increase in temperature is suppressed and makes it possible to prevent adverse effects on the solid-state battery itself and on a device using the solid-state battery.

For the PTC resistor layer containing the electroconductive material and the polymer, the polymer is deformed and fluidized when pressure is applied to the solid-state battery, whereby the PTC resistor layer cannot maintain its structure and, as a result, may fail to exert the PTC resistor function. In Patent Literature 1, for the purpose of allowing the PTC resistor layer to maintain its structure even when pressure is applied to the solid-state battery, the PTC resistor layer that further contains an insulating inorganic substance, which is generally said to have high strength, is disclosed. It was thought that inside the PTC resistor layer, electronic resistance is increased by the insulating inorganic substance, thereby increasing electronic resistance in the whole electrode.

However, as a result of research, it was found that in the electrode comprising the PTC resistor layer containing the insulating inorganic substance, the electronic resistance inside the PTC resistor layer is higher than expected. This seems to be because, since the PTC resistor layer contains the insulating inorganic substance, many pores are present inside the PTC resistor layer.

For the electrode for solid-state batteries according to the disclosed embodiments, the porosity of the PTC resistor layer is in the specific value range, whereby when the electrode is used in a solid-state battery, a decrease in the performance of the solid-state battery can be suppressed.

The electrode for solid-state batteries according to the disclosed embodiments comprises an electrode active material layer, a current collector and a PTC resistor layer.

FIG. 1 is a view showing an example of the layer structure of the electrode for solid-state batteries according to the disclosed embodiments, and it is also a schematic cross sectional view of the electrode along the laminating direction. As shown in FIG. 1, an electrode 10 for solid-state batteries according to the disclosed embodiments, comprises an electrode active material layer 2, a current collector 3, and a PTC resistor layer 1 disposed between the electrode active material layer 2 and the current collector 3.

Hereinafter, these layers of the electrode for solid-state batteries will be described in detail.

(1) PTC Resistor Layer

The PTC resistor layer is a layer which contains an electroconductive material, an insulating inorganic substance and a polymer, and which is disposed between the electrode active material layer and the current collector.

The electroconductive material contained in the PTC resistor layer is not particularly limited, as long as it has electroconductivity. As the electroconductive material, examples include, but are not limited to, carbon-containing electroconductive materials such as carbon black, activated carbon, carbon fiber (e.g., carbon nanotube, carbon nanofiber) and graphite. The electroconductive material contained in the PTC resistor layer may be carbon black. The electroconductive material may be in a particulate form. As the particulate form, examples include, but are not limited to, a fibrous form.

The volume ratio of the electroconductive material in the PTC resistor layer is not particularly limited. When the total volume of the electroconductive material, the insulating inorganic substance and the polymer is determined as 100 volume %, the volume ratio of the electroconductive material in the PTC resistor layer may be from 7 volume % to 50 volume %, or it may be from 7 volume % to 10 volume %.

The insulating inorganic substance contained in the PTC resistor layer functions to suppress deformation and fluidization of the PTC resistor layer in the electrode for solid-state batteries, both of which are due to high temperature and pressure.

The insulating inorganic substance is not particularly limited, as long as it is a material that has a higher melting point than the below-described polymer. As the insulating inorganic substance, examples include, but are not limited to, a metal oxide and a metal nitride. As the metal oxide, examples include, but are not limited to, alumina, zirconia and silica. As the metal nitride, examples include, but are not limited to, a silicon nitride. Also, as the insulating inorganic substance, examples include, but are not limited to, a ceramic material. The insulating inorganic substance may be a metal oxide.

In general, the insulating inorganic substance is in a particulate form. The insulating inorganic substance may be primary particles or secondary particles.

The average particle diameter (D₅₀) of the insulating inorganic substance may be from 0.2 μm to 5 μm, or it may be from 0.4 μm to 2 μm, for example. The distribution of the insulating inorganic substance particles is not particularly limited. The distribution of the particles may be a normal distribution when it is represented by a frequency distribution.

The volume ratio of the insulating inorganic substance in the PTC resistor layer is not particularly limited. When the total volume of the electroconductive material, the insulating inorganic substance and the polymer is determined as 100 volume %, the volume ratio of the insulating inorganic substance in the PTC resistor layer may be from 40 volume % to 85 volume %, or it may be from 50 volume % to 60 volume %.

When the volume ratio of the insulating inorganic substance in the PTC resistor layer is too small, it may be difficult to sufficiently suppress the deformation and fluidization of the PTC resistor layer, both of which are due to heating and pressure. On the other hand, when the volume ratio of the insulating inorganic substance in the PTC resistor layer is too large, the volume ratio of the polymer is relatively small. As a result, the effect of separating the particles of the electroconductive material by the polymer may be insufficiently exerted, and an increase in electronic resistance may be insufficient. Also when the volume ratio of the insulating inorganic substance in the PTC resistor layer is too large, electroconductive paths, which are formed by the electroconductive material, may be blocked by the insulating inorganic substance, and the electron conductivity of the PTC resistor layer during normal use may decrease. In the disclosed embodiments, the electron conductivity of the PTC resistor layer means the property of conducting electrons through the PTC resistor layer, and it is strictly different from the electroconductivity of the PTC resistor layer (the property of conducting electricity through the PTC resistor layer).

The porosity of the PTC resistor layer is generally from 5% to 13%. It may be from 5% to 12%, or it may be from 5% to 11%.

As will be shown below in Comparative Example 1, for a conventional solid-state battery, the porosity of the PTC resistor layer was more than 30%. In the disclosed embodiments, by using the PTC resistor layer having a porosity which is smaller than ever before and which is in the specific value range, the electron conductivity of the PTC resistor layer is increased higher than ever before. It is presumed that this is because, for the PTC resistor layer having a higher density than ever before, the number of the electron-conductive paths inside the layer is larger than ever before.

Accordingly, by using the PTC resistor layer having such excellent electron conductivity, a decrease in the performance of the solid-state battery can be suppressed.

When the porosity is more than 13%, the number of the electron-conductive paths inside the PTC resistor layer is too small. Accordingly, the electron conductivity of the PTC resistor layer deteriorates. On the other hand, it is possible to produce the PTC resistor layer having a porosity of less than 5%. However, it is not easy due to the following reason. At least three different materials (the electroconductive material, the insulating inorganic substance and the polymer) are needed to form the PTC resistor layer. Even if the materials are mixed as well as possible by use of a solvent, etc., it is difficult to prepare an absolutely uniform mixture, and it is inevitable to avoid pore formation in the dried PTC resistor layer.

The porosity of the PTC resistor layer can be controlled to 5% to 13% by a pressing method such as roll pressing (which will be described below), cold isostatic pressing (CIP) and hot isostatic pressing (HIP).

In general, it is often difficult to control the porosity of the PTC resistor layer to 5% to 13% by confining the solid-state battery with the below-described confining member. This is because the pressure applied by the confining member to the solid-state battery is only enough to closely attach the layers constituting the solid-state battery, and the pressure is not enough to change the internal structure of the PTC resistor layer and to reduce the pores inside the layer. Also for the conventional solid-state battery, the porosity of the PTC resistor layer is more than 30% (see Comparative Example 1).

Accordingly, even if the conventional solid-state battery is combined with the confining member, it is inconceivable that the porosity of the PTC resistor layer is decreased to 5% to 13%.

Also, the porosity of the PTC resistor layer can be controlled to 5% to 13% by methods other than the pressing method, such as drying under reduced pressure and heating.

In the case of forming the PTC resistor layer through one-time application of the slurry, the method for calculating the porosity is as follows. First, for the laminate of the PTC resistor layer and the current collector (hereinafter, the laminate may be referred to as “laminate A”) and for a current collector that is the same type as the current collector used to produce the laminate A, the mass and the thickness are measured as follows.

The mass is measured by the following method.

First, the laminate A and the current collector are each cut into an area of 1 cm². The mass M₁ of the cut laminate A and the mass M₀ of the cut current collector are measured by use of an analytical balance (product name: GR-202, manufactured by: A&D Co., Ltd.)

The thickness is measured by the following method.

The thickness T₁ of the cut laminate A and the thickness T₀ of the cut current collector are measured by use of a thickness gauge (product name: PG-01J, manufactured by: Teclock Corporation, probe: ZS-579).

Next, the density of the PTC resistor layer is calculated by the following method.

The density D (g/cm³) of the PTC resistor layer is obtained by the following formula (X), using the true densities (g/cm³) of the materials used to form the PTC resistor layer (that is, the electroconductive material, the insulating inorganic substance and the polymer) and the volume ratios (%) of the materials in the PTC resistor layer.

D=(TD ₁ ×V ₁ +TD ₂ ×V ₂ TD ₃ ×V ₃)/100  Formula (X)

where D is the density (g/cm³) of the PTC resistor layer; TD₁ is the true density (g/cm³) of the electroconductive material; V₁ is the volume ratio (%) of the electroconductive material; TD₂ is the true density (g/cm³) of the insulating inorganic substance; V₂ is the volume ratio (%) of the insulating inorganic substance; TD₃ is the true density (g/cm³) of the polymer; and V₃ is the volume ratio (%) of the polymer.

The porosity P (%) of the PTC resistor layer is calculated by the following formula (Y), using the above-described thickness, mass and density values.

P=[(M ₁ −M ₀)/{(T ₁ −T ₀)×1 cm² ×D}]×100  Formula (Y)

where P is the porosity (%) of the PTC resistor layer; M₁ is the mass (g) of the laminate A; M₀ is the mass (g) of the current collector; T₁ is the thickness (cm) of the laminate A; T₀ is the thickness (cm) of the current collector; and D is the density (g/cm³) of the PTC resistor layer.

In the case of forming the PTC resistor layer through, like the below-described modified example of forming the PTC resistor layer, two-time application of the slurry, first, the porosities of the below-described first and second coating layers are calculated. Then, the weighted average of the two porosities is calculated, and the thus-obtained value is determined as the porosity of the PTC resistor layer.

The detailed method for calculating the porosity is as follows.

For the laminate A (the laminate of the PTC resistor layer formed by two-time application of the slurry and the current collector), for a laminate obtained by one-time application of the slurry to a surface of the current collector (hereinafter, the laminate may be referred to as “laminate a”) and for the current collector that is the same type as the one used to produce the laminate A, the mass and the thickness are measured. The method for measuring the mass and the method for measuring the thickness are the same as the above-described methods using the analytical balance and the thickness gauge.

The method for calculating the density of each layer formed by slurry application, follows the formula (X). In this case, the variables (TD₁, V₁, TD₂, V₂, TD₃ and V₃) of the formula (X) are obtained from the components and composition of each slurry. Hereinafter, the density of the first coating layer and that of the second coating layer are defined ad D¹ and D², respectively.

The porosity P¹ (%) of the first coating layer is calculated by the following formula (Y1), using the above-described thickness, mass and density values.

P ¹=[(M ₂ −M ₀)/{(T ₂ −T ₀)×1 cm² ×D ¹}]×100  Formula (Y1)

where P¹ is the porosity (%) of the first coating layer; M₂ is the mass (g) of the laminate a; M₀ is the mass (g) of the current collector; T₂ is the thickness (cm) of the laminate a; T₀ is the thickness (cm) of the current collector; and D¹ is the density (g/cm³) of the first coating layer.

The porosity P² (%) of the second coating layer is calculated by the following formula (Y2), using the above-described thickness, mass and density values.

P ²=[(M ₁ −M ₂)/{(T ₁ −T ₂)×1 cm² ×D ²}]×100  Formula (Y2)

where P² is the porosity (%) of the second coating layer; M₁ is the mass (g) of the laminate A; M₂ is the mass (g) of the laminate a; T₁ is the thickness (cm) of the laminate A; T₂ is the thickness (cm) of the laminate a; and D² is the density (g/cm³) of the second coating layer.

The weighted average of the above-described two porosities is calculated by the following formula (Y3), and the thus-obtained value (P) is determined as the porosity of the PTC resistor layer.

P=(P ¹×(T ₂ −T ₀)+P ²×(T ₁ −T ₂))/(T ₁ −T ₀)  Formula (Y3)

where P is the porosity (%) of the PTC resistor layer; P¹ is the porosity (%) of the first coating layer; T₂ is the thickness (cm) of the laminate a; T₀ is the thickness (cm) of the current collector; P² is the porosity (%) of the second coating layer; and T₁ is the thickness (cm) of the laminate A.

Even in the case of forming the PTC resistor layer through three-time application of the slurry, the porosity of the PTC resistor layer can be obtained in the same manner as the case of forming the PTC resistor layer through two-time application of the slurry. More specifically, the porosities of the layers formed by the slurry application are calculated, and the weighted average is determined as the porosity of the PTC resistor layer.

Also, the porosity of the PTC resistor layer can be measured from the solid-state battery thus formed or from the electrode taken out therefrom. In this case, as the measuring method, examples include, but are not limited to, image analysis.

As the method for measuring the porosity by image analysis, examples include, but are not limited to, the following method: a cross section of the electrode containing the PTC resistor layer or a cross section of the PTC resistor layer itself is processed by, for example, a cross section polisher (CP) and/or focused ion beam (FIB); a scanning electron microscopy image (SEM image) of the cross section is taken; and the porosity is measured from the image. In the case of calculating the porosity from the cross section of the electrode, the thickness of the PTC resistor layer is determined as the distance between the electrode active material layer and the current collector.

The polymer contained in the PTC resistor layer is not particularly limited, as long as it is a polymer that expands when its temperature exceeds its melting point by heating. As the polymer, examples include, but are not limited to, thermoplastic resins such as polypropylene, polyethylene, polyvinyl chloride, polyvinylidene fluoride (PVDF), polyfluoroethylene, polystyrene, ABS resin, methacryl resin, polyamide, polyester, polycarbonate and polyacetal. These polymers may be used alone or in combination of two or more kinds.

From the viewpoint of melting point and ease of processing, the polymer may be a fluorine-containing polymer such as polyvinylidene fluoride and polyfluoroethylene, or it may be polyethylene. The polymer may be polyvinylidene fluoride.

The volume ratio of the polymer in the PTC resistor layer is not particularly limited. When the total volume of the electroconductive material, the insulating inorganic substance and the polymer is determined as 100 volume %, the volume ratio of the polymer in the PTC resistor layer may be from 8 volume % to 60 volume %, or it may be from 8 volume % to 45 volume %.

The thickness of the PTC resistor layer is not particularly limited. It may be from about 1 μm to about 30 μm.

(2) Electrode Active Material Layer

The electrode active material layer is not particularly limited, as long as it contains at least an electrode active material. As needed, it may contain a binder, an electroconductive material, and a solid electrolyte.

In the case of using the electrode for solid-state batteries according to the disclosed embodiments as the cathode, the electrode active material is not particularly limited, as long as it is an electrode active material that is generally used as a cathode active material. For example, when transferred ions are lithium ions, as the cathode active material, examples include, but are not limited to, a compound having a layered structure (such as LiCoO₂ and LiNiO₂), a compound having a spinel-type structure (such as LiMn₂O₄), and a compound having an olivine-type structure (such as LiFePo₄).

In the case of using the electrode for solid-state batteries according to the disclosed embodiments as the anode, the electrode active material is not particularly limited, as long as it is an electrode active material that is generally used as an anode active material. For example, when the transferred ions are lithium ions, as the anode active material, examples include, but are not limited to, a carbonaceous material, a lithium alloy, an oxide and a nitride.

The binder is not particularly limited, as long as it is chemically and electrically stable. As the binder, examples include, but are not limited to, a fluorine-containing binder such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

The electroconductive material is not particularly limited, as long as it has electroconductivity. As the electroconductive material, examples include, but are not limited to, carbonaceous materials such as carbon black, activated carbon, carbon fiber (e.g., carbon nanotube, carbon nanofiber) and graphite.

The material for the solid electrolyte is not particularly limited, as long as it has ion conductivity. As the material, examples include, but are not limited to, inorganic materials such as a sulfide material and an oxide material. As the sulfide material, examples include, but are not limited to, Li₂S—SiS₂, LiI—Li₂S—SiS₂, LiI—Li₂S—P₂S₅, LiI—Li₂O—Li₂S—P₂S₅, LiI—Li₂S—P₂O₅, LiI—Li₃PO₄—P₂S₅, Li₂S—P₂S₅, Li₃PS₄, LiI—LiBr—Li₂S—P₂S₅ and Li₂S—P₂S₅—GeS₂.

(3) Current Collector

The material for the current collector is not particularly limited, as long as it has electron conductivity. As the material for the current collector, examples include, but are not limited to, Al, Cu, Ni, SUS and Fe. In the case of using the electrode for solid-state batteries according to the disclosed embodiments as the cathode, Al may be used as the material for the current collector. In the case of using the electrode for solid-state batteries according to the disclosed embodiments as the anode, Cu may be used as the material for the current collector.

(4) Method for Producing the Electrode for Solid-State Batteries

The method for producing the electrode for solid-state batteries is not particularly limited, as long as the above-described electrode for solid-state batteries can be obtained by the method. Hereinafter, two embodiments of the method for producing the electrode for solid-state batteries will be described. The method for producing the electrode for solid-state batteries according to the disclosed embodiments, is not limited to the two embodiments.

A. First Embodiment

The first embodiment of the method for producing the electrode for solid-state batteries comprises (a) forming the PTC resistor layer on one surface of the current collector and (b) laminating the electrode active material layer on the PTC resistor layer.

(a) Forming the PTC Resistor Layer on One Surface of the Current Collector

This is a step of forming the PTC resistor layer by applying the first slurry to one surface of the current collector and drying the applied first slurry.

The first slurry contains an electroconductive material, an insulating inorganic substance and a polymer. Details of the materials are as described above. The content ratio of the electroconductive material, the insulating inorganic substance and the polymer in the first slurry and in the below-described second slurry, may be appropriately determined so as to correspond to the volume ratio and distribution of the electroconductive material, the insulating inorganic substance and the polymer in the PTC resistor layer of the electrode for solid-state batteries.

For the content ratio of the materials in the first slurry, the electroconductive material, the polymer and the insulating inorganic substance may be at a volume ratio of 10:30:60, for example.

The first slurry may contain a non-aqueous solvent for dissolving or dispersing the electroconductive material, the insulating inorganic substance and the polymer. The type of the non-aqueous solvent is not particularly limited. As the non-aqueous solvent, examples include, but are not limited to, N-methylpyrrolidone, acetone, methyl ethyl ketone and dimethylacetamide. From the viewpoint of safety such as high flash point, small influence on human body and so on, the non-aqueous solvent may be N-methylpyrrolidone.

The content ratio of the non-aqueous solvent in the first slurry is not particularly limited. When the total volume of the first slurry is determined as 100 volume %, the non-aqueous solvent may be from 80 volume % to 93 volume %, or it may be from 82 volume % to 90 volume %.

The method for forming the PTC resistor layer is not particularly limited. Hereinafter, a typical example and a modified example of the method for forming the PTC resistor layer, will be described. However, the method for forming the PTC resistor layer is not limited to the two examples.

In the typical example of the method for forming the PTC resistor layer, the first slurry in which the electroconductive material, the insulating inorganic substance and the polymer are dispersed in the non-aqueous solvent, is applied onto the current collector, and the applied slurry is dried. To uniformly form the PTC resistor layer, the solid content concentration of the first slurry containing the electroconductive material, the insulating inorganic substance and the polymer, may be from 13 mass % to 40 mass %.

The thickness of the PTC resistor layer is not particularly limited. It may be from about 1 μm to about 30 μm.

The condition for drying the first slurry is not particularly limited. For example, it may be a temperature condition in which the non-aqueous solvent can be distilled away.

In the modified example of the method for forming the PTC resistor layer, the PTC resistor layer is formed by further applying the second slurry to the surface of the layer formed by use of the first slurry (hereinafter, it may be referred to as “first coating layer”). In this case, the PTC resistor layer is a layer comprising the solid content of the second slurry and the first coating layer.

The second slurry contains an electroconductive material and a polymer. The second slurry may further contain an insulating inorganic substance. When the insulating inorganic substance is not contained in the second slurry, contact between the PTC resistor layer and the electrode active material layer can be better compared to the case where the second slurry contains the insulating inorganic substance.

For the content ratio of the materials in the second slurry, in the case where the insulating inorganic substance is not contained in the second slurry, the electroconductive material and the polymer may be at a volume ratio of from 85:15 to 20:80, for example.

The content ratio of the non-aqueous solvent in the second slurry is not particularly limited. When the total volume of the second slurry is determined as 100 volume %, the non-aqueous solvent may be from 75 volume % to 95 volume %, or it may be from 85 volume % to 90 volume %.

The method for applying and drying the second slurry is not particularly limited. In general, the second slurry in which the electroconductive material and the polymer are dispersed in the non-aqueous solvent, is applied onto the current collector, and the applied slurry is dried. To uniformly apply the second slurry, the solid content concentration of the second slurry containing at least the electroconductive material and the polymer, may be from 13 mass % to 35 mass %.

The thickness of the layer corresponding to the part formed by applying and drying the second slurry (hereinafter, the layer may be referred to as “second coating layer”) is not particularly limited. The thickness may be from about 1 μm to about 10 μm, or it may be from about 2 μm to about 6 μm. The thickness of the second coating layer is obtained from, for example, a difference between the thickness of the laminate before the second coating layer is formed and the thickness of the laminate after the second coating layer is formed.

In general, after the second slurry is applied and dried, the first coating layer and the solid content of the second slurry are combined to form the PTC resistor layer.

Before laminating the electrode active material layer on the PTC resistor layer, the laminate of the current collector and the PTC resistor layer may be pressed. The laminate may be pressed by roll pressing, cold isostatic pressing (CIP), hot isostatic pressing (HIP), etc. When the applied pressing pressure is too high, the PTC resistor layer may be cracked. For example, in the case of roll pressing, the pressing pressure may be a line pressure of from 5.6 kN/cm to 22.4 kN/cm.

By pressing the laminate of the current collector and the PTC resistor layer, the porosity of the PTC resistor layer can be controlled to 5% to 13% (see Examples 1 to 3).

(b) Laminating the Electrode Active Material Layer on the PTC Resistor Layer

By laminating the electrode active material layer on the PTC resistor layer, a laminate of the electrode active material layer, the PTC resistor layer and the current collector is produced. Details of the materials that can be used to form the electrode active material layer (an electrode active material, a binder and a solid electrolyte) are as described above.

As the method for forming the electrode active material layer, a known technique may be used. For example, the electrode active material layer can be formed as follows: a mixture of raw materials for the electrode active material layer is stirred well; the raw material mixture is applied onto a substrate or onto the PTC resistor layer; and the applied raw material mixture is appropriately dried, thereby forming the electrode active material layer.

In the case of forming the electrode active material layer on a substrate, roll pressing in a high temperature condition (hot roll pressing) may be used. By hot roll pressing, the electrode active material layer thus obtained can be more densified. In the case of forming the electrode active material layer on the PTC resistor layer, if the heating temperature of the hot roll pressing is too high, there is a possibility that the polymer in the PTC resistor layer is thermally expanded. Accordingly, it is needed to determine the upper limit temperature of the hot roll pressing, depending on the properties of the polymer, the composition of the PTC resistor layer, etc. In general, the hot roll pressing may be carried out at a temperature less than the melting point of the polymer.

B. Second Embodiment

The second embodiment of the method for producing the electrode for solid-state batteries, comprises (a) forming the first coating layer on one surface of the current collector, (b′) forming the second coating layer on one surface of the electrode active material layer, and (c′) producing a laminate of the current collector, the PTC resistor layer and the electrode active material layer.

Of them, (a) is the same as the first embodiment described above. Hereinafter, (b′) and (c′) will be described.

(b′) Forming the Second Coating Layer on One Surface of the Electrode Active Material Layer

This is a step of forming the second coating layer on the electrode active material layer by applying the second slurry to one surface of a substrate, drying the applied second slurry to form the second coating layer, and then transferring the second coating layer from the substrate to the electrode active material layer.

In the first embodiment, as described above in (a), the second coating layer is formed on the first coating layer. In this step of the second embodiment, the second coating layer is formed on the electrode active material layer. As just described, the two embodiments differ in the member on which the second coating layer is formed.

Transferring the second coating layer from the substrate to the electrode active material layer, is advantageous in that the solvent used in the second slurry has no influence on the electrode active material layer.

The second slurry and the thus-obtained second coating layer are the same as those of the first embodiment.

The substrate used to form the second coating layer is not particularly limited. For example, Al, PET, Cu, SUS or the like may be used.

(c′) Producing the Laminate of the Electrode Active Material Layer, the PTC Resistor Layer and the Current Collector

In this step, the current collector and the electrode active material layer are laminated so that the first coating layer of the current collector and the second coating layer of the electrode active material layer are in contact with each other, whereby the first coating layer and the second coating layer are combined to form the PTC resistor layer. As a result, the laminate of the electrode active material layer, the PTC resistor layer and the current collector is formed.

(5) Measurement of the Electronic Resistance of the Electrode for Solid-State Batteries

An evaluation item of the electrode for solid-state batteries is electronic resistance measurement. For the electronic resistance measurement, a solid-state battery comprising the electrode for solid-state batteries or an evaluation sample comprising the electrode for solid-state batteries, is used.

Hereinafter, the evaluation sample will be described. FIG. 3 is a schematic cross-sectional view of an evaluation sample including the electrode for solid-state batteries according to the disclosed embodiments. An electrode 10 for solid-state batteries shown in FIG. 3 corresponds to the electrode 10 for solid-state batteries shown in FIG. 1 and to an electrode 10 for solid-state batteries shown in FIG. 2.

As shown in FIG. 3, the layer structure of an evaluation sample 50 is as follows: current collector 3/PTC resistor layer 1/cathode active material layer 2/current collector 3′/cathode active material layer 2/PTC resistor layer 1/current collector 3. As is clear from FIG. 3, the evaluation sample 50 is formed by disposing the current collector 3′ between the cathode active material layers 2 of the two electrodes 10 for solid-state batteries, the layers facing each other.

An example of the method for producing the evaluation sample is as follows. First, two laminates of the PTC resistor layer and the current collector (hereinafter, each laminate may be referred to as laminate A) and two laminates of the cathode active material layer and the current collector (hereinafter, each laminate may be referred to as laminate B) were produced. Next, the two laminates B are laminated so that the cathode active material layer of one laminate B and the current collector of the other laminate B are in contact with each other. From a laminate thus obtained, the current collector disposed outside is peeled off, thereby producing a laminate having the following layer structure: cathode active material layer/current collector/cathode active material layer (hereinafter, the laminate may be referred to as laminate C). The laminate C corresponds to the central part (cathode active material layer 2/current collector 3′/cathode active material layer 2) of the evaluation sample 50 shown in FIG. 3. Finally, the two laminates A are laminated on both surfaces of the laminate C so that the cathode active material layers are in contact with the PTC resistor layers, thereby producing the evaluation sample 50 shown in FIG. 3.

FIG. 3 is a schematic view of a circuit for electronic resistance measurement, which includes an evaluation sample. As shown in FIG. 3, a tester 40 is connected to the evaluation sample 50, thereby producing a circuit 200 for electronic resistance measurement. The electronic resistance of the evaluation sample 50 in a room temperature condition (e.g., 25° C.) and the electronic resistance thereof in a high temperature condition (e.g., 250° C.) can be measured by use of the circuit 200 for electronic resistance measurement.

In place of the evaluation sample 50 shown in FIG. 3, a solid-state battery described below may be used for the electronic resistance measurement.

FIG. 4 is a view showing a relationship between the electronic resistance of an evaluation sample including a PTC resistor layer and the resistance of a solid-state battery comprising an electrode including the PTC resistor layer. FIG. is a graph with the resistance (Ω·cm²) of the solid-state battery on the vertical axis and the electronic resistance (Ω·cm²) of the evaluation sample on the horizontal axis.

As is clear from FIG. 4, the resistance of the solid-state battery increases as the electronic resistance of the evaluation sample increases. As just described, since the electronic resistance of the evaluation sample and the resistance of the solid-state battery are highly correlated with each other, the result of the electronic resistance measurement of the evaluation sample can be said to be a test result reflecting the performance of the solid-state battery itself.

2. Solid-State Battery

The solid-state battery of the disclosed embodiments is a solid-state battery comprising a cathode, an anode and an electrolyte layer disposed between the cathode and the anode, wherein at least one of the cathode and the anode is the above-mentioned electrode for solid-state batteries.

In the disclosed embodiments, the solid-state battery means a battery containing a solid electrolyte. Accordingly, as long as the solid-state battery of the disclosed embodiments contains a solid electrolyte, the solid-state battery may be fully composed of a solid component, or it may contain both a solid component and a liquid component.

FIG. 2 is a view showing an example of the layer structure of the solid-state battery of the disclosed embodiments, and it is also a schematic cross sectional view of the solid-state battery along the laminating direction. As shown in FIG. 2, a solid-state battery 100 comprises the electrode 10 for solid-state batteries, an opposite electrode 30, and an electrolyte layer 20 disposed between the electrode 10 for solid-state batteries and the opposite electrode 30.

The electrode 10 for solid-state batteries corresponds to the above-described electrode for solid-state batteries according to the disclosed embodiments. The opposite electrode is an electrode facing the electrode 10 for solid-state batteries. The electrode 10 for solid-state batteries and the opposite electrode 30 may be the cathode and the anode, respectively; the electrode 10 for solid-state batteries and the opposite electrode 30 may be the anode and the cathode, respectively; or, unlike FIG. 2, each of the cathode and the anode may be the electrode for solid-state batteries according to the disclosed embodiments.

The electrode 10 for solid-state batteries is as described above. The opposite electrode 30, that is, a cathode or anode that is generally used in the solid-state battery, may be selected from known techniques. Especially, the cathode active material layer and cathode current collector which can be used in the cathode, and the anode active material layer and anode current collector which can be used in the anode, may be appropriately selected from the above-described materials used in the disclosed embodiments.

The electrolyte layer 20 is not particularly limited, as long as it is a layer having ion conductivity. The electrolyte layer 20 may be a layer composed of a solid electrolyte only, or it may be a layer containing both a solid electrolyte and a liquid electrolyte.

As the electrolyte layer composed of the solid electrolyte only, examples include, but are not limited to, a polymer solid electrolyte layer, an oxide solid electrolyte layer and a sulfide solid electrolyte layer.

As the electrolyte layer containing both the solid electrolyte and the liquid electrolyte, examples include, but are not limited to, a porous solid electrolyte layer impregnated with an aqueous or non-aqueous electrolyte solution.

The form of the solid-state battery of the disclosed embodiments is not particularly limited. As the form of the solid-state battery, examples include, but are not limited to, common forms such as a coin form, a flat plate form and a cylindrical form.

The solid-state battery of the disclosed embodiments may be a single cell as shown in FIG. 2, or it may be an assembly of the single cells. As the cell assembly, examples include, but are not limited to, a cell stack composed of a stack of single cells in a flat plate form.

As described above, in the pressure-applied condition, the electrode for solid-state batteries according to the disclosed embodiments exerts the excellent effect of suppressing a decrease in solid-state battery performance. Accordingly, the electrode for solid-state batteries according to the disclosed embodiments exerts the excellent effect even when an unintentional pressure is applied (such as occurrence of a failure in the solid-state battery due to an internal short circuit, overcharging, etc.) or even when an intentional pressure is applied (such as use of a confining member in combination with the solid-state battery). In general, a failure occurs in the solid-state battery when an unexpected local pressure is applied to the solid-state battery. Also, in the case of using the confining member in combination with the solid-state battery, a predetermined pressure is generally applied to the whole solid-state battery.

The confining member may be a member that can apply a confining pressure to the laminate of the two electrodes and the electrolyte layer disposed therebetween, in the approximately parallel direction to the laminating direction. A known solid-state battery confining member may be used in combination with the solid-state battery of the disclosed embodiments. As the known solid-state battery confining member, examples include, but are not limited to, a confining member comprising a pair of plates that are used to sandwich the solid-state battery, one or more rods that are used to connect the two plates, and a controller that is connected to the rod(s) and used to control the confining pressure by use of a screw structure, etc. In this case, the confining pressure applied to the solid-state battery can be controlled by appropriately controlling the controller.

The confining pressure is not particularly limited. It may be 0.1 MPa or more, may be 1 MPa or more, or may be 5 MPa or more. When the confining pressure is 0.1 MPa or more, the layers constituting the solid-state battery are in better contact with each other. On the other hand, the confining pressure may be 100 MPa or less, may be 50 MPa or less, or may be 20 MPa or less, for example. When the confining pressure is 100 MPa or less, it is not needed to use the special confining member.

EXAMPLES

Hereinafter, the disclosed embodiments will be further clarified by the following examples. The disclosed embodiments are not limited to the following examples, however.

1. Production of an Evaluation Sample Example 1

(1) Production of a Laminate of a PTC Resistor Layer and an Aluminum Foil

The following materials for a first slurry were prepared.

-   -   Electroconductive material: Furnace black (manufactured by:         Tokai Carbon Co., Ltd., average primary particle diameter: 66         nm)     -   Insulating inorganic substance: Alumina (product name: CB-P02,         manufactured by: Showa Denko K. K., average particle diameter         (D₅₀): 2 μm)     -   Polymer: PVDF (product name: KF POLYMER L #9130, manufactured         by: Kureha Corporation)     -   Non-aqueous solvent: N-methylpyrrolidone

The furnace black, the PVDF and the alumina were mixed at a volume ratio of 10:30:60 to prepare a mixture. The N-methylpyrrolidone was added to the mixture, thereby producing the first slurry. Then, the first slurry was applied on an aluminum foil having a thickness of 15 μm (a current collector). The applied first slurry was dried in a stationary drying oven at 100° C. for one hour, thereby forming a PTC resistor layer having a thickness of 10 μm.

The above step was carried out twice to produce two laminates of the PTC resistor layer and the aluminum foil (laminates A). The laminates A were subjected to roll pressing under the conditions of a line pressure of 5.6 kN/cm and room temperature.

(2) Production of a Laminate of a Cathode Active Material Layer and an Aluminum Foil

The following materials for the cathode active material layer were put in a container to obtain a mixture.

-   -   Cathode active material: LiNi_(1/3)Co_(1/3)O₂ particles (average         particle diameter: 6 μm)     -   Sulfide-based solid electrolyte: Li₂S—P₂S₅-based glass ceramic         particles containing LiI and LiBr (average particle diameter:         0.8 μm)     -   Electroconductive material: VGCF     -   Binder: A 5 mass % solution of a PVDF-based binder in butyl         butyrate

The mixture in the container was stirred by an ultrasonic disperser (product name: UH-50, manufactured by: SMT Co., Ltd.) for 30 seconds. Next, the container was shaken by a shaking device (product name: TTM-1, manufactured by: Sibata Scientific Technology Ltd.) for three minutes. The mixture in the container was further stirred by the ultrasonic disperser for 30 seconds, thereby preparing a slurry for forming the cathode active material layer.

Using an applicator, the slurry for forming the cathode active material layer was applied to one surface of an aluminum foil (serving as a cathode current collector, manufactured by Showa Denko K. K.) by the doctor blade method. The applied slurry was dried on a hot plate at 100° C. for 30 minutes, thereby forming the cathode active material layer on one surface of the aluminum foil.

The above step was carried out twice to produce two laminates of the cathode active material layer and the aluminum foil (laminates B).

(3) Production of an Evaluation Sample

First, a laminate C was produced by use of the two laminates B. The laminate C had the following layer structure: cathode active material layer/aluminum foil/cathode active material layer. Details are as follows.

The two laminates B were laminated so that the cathode active material layer of one laminate B and the aluminum foil of the other laminate B were in contact with each other. A laminate thus obtained was subjected to roll pressing at 10 kN/cm in a room temperature condition, thereby obtaining a laminate having the following layer structure: cathode active material layer/aluminum foil/cathode active material layer/aluminum foil. The aluminum foil disposed outside the laminate was peeled off from the laminate. The laminate was subjected to roll pressing at 50 kN/cm at 165° C. to densify the two cathode active material layers, thereby obtaining a laminate having the following layer structure: cathode active material layer/aluminum foil/cathode active material layer (the laminate C).

The laminate C was disposed between the laminates A and they were laminated so that the cathode active material layers of the laminate C were in contact with the PTC resistor layers of the laminates A, thereby obtaining the evaluation sample of Example 1 having the following layer structure: aluminum foil/PTC resistor layer/cathode active material layer/aluminum foil/cathode active material layer/PTC resistor layer/aluminum foil.

A cross section of the evaluation sample of Example 1 was the same as the evaluation sample 50 shown in FIG. 3. As shown in FIG. 3, the layer structure of the evaluation sample was as follows: current collector 3 (aluminum foil)/PTC resistor layer 1/cathode active material layer 2/current collector 3′ (aluminum foil)/cathode active material layer 2/PTC resistor layer 1/current collector 3 (aluminum foil). As is clear from FIG. 3, the evaluation sample 50 was formed by disposing the current collector 3′ (aluminum foil) between the two electrodes 10 for solid-state batteries.

Example 2

The evaluation sample of Example 2 was produced in the same manner as Example 1, except that in “(1) Production of a laminate of a PTC resistor layer and an aluminum foil”, the roll pressing pressure applied to the laminates A was changed from 5.6 kN/cm to 14.2 kN/cm.

Example 3

The evaluation sample of Example 3 was produced in the same manner as Example 1, except that in “(1) Production of a laminate of a PTC resistor layer and an aluminum foil”, the roll pressing pressure applied to the laminates A was changed from 5.6 kN/cm to 22.4 kN/cm.

Comparative Example 1

The evaluation sample of Comparative Example 1 was produced in the same manner as Example 1, except that in “(1) Production of a laminate of a PTC resistor layer and an aluminum foil”, the laminates A were not subjected to roll pressing.

2. Evaluation of the Evaluation Samples

The evaluation samples of Examples 1 to 3 and Comparative Example 1 were evaluated as follows. The results are shown in Table 1.

(1) Calculation of Porosity

For the laminates A used to produce the evaluation samples (the laminates of the PTC resistor layer and the aluminum foil) and for an aluminum foil that is the same type as the aluminum foil used to produce the laminates A, the mass and the thickness were measured as follows, and then the density was also calculated as follows.

a. Mass

The laminate A and the aluminum foil were each cut into an area of 1 cm². The mass m₁ of the cut laminate A and the mass m₀ of the cut aluminum foil were measured by use of an analytical balance (product name: GR-202, manufactured by: A&D Co., Ltd.)

b. Thickness

The thickness t₁ of the cut laminate A and the thickness t₀ of the cut aluminum foil were measured by use of a thickness gauge (product name: PG-01J, manufactured by: Teclock Corporation, probe: ZS-579).

c. Density

The density d (g/cm³) of the PTC resistor layer was obtained by the following formula (x), using the true densities (g/cm³) of the following three materials used to form the PTC resistor layer and the volume ratios (%) of the materials in the PTC resistor layer.

-   -   Furnace black (manufactured by: Tokai Carbon Co., Ltd., average         primary particle diameter: 66 nm)     -   Alumina (product name: CB-P02, manufactured by: Showa Denko K.         K., average particle diameter (D₅₀): 2 μm)     -   PVDF (product name: KF POLYMER L #9130, manufactured by: Kureha         Corporation)

d=(td ₁ ×v ₁ +td ₂ ×v ₂ +td ₃ ×v ₃)/100  Formula (x)

where d is the density (g/cm³) of the PTC resistor layer; td₁ is the true density (g/cm³) of the furnace black; v₁ is the volume ratio (%) of the furnace black; td₂ is the true density (g/cm³) of the alumina; v₂ is the volume ratio (%) of the alumina; td₃ is the true density (g/cm³) of the PVDF; and v₃ is the volume ratio (%) of the PVDF.

d. Porosity

The porosity p (%) of the PTC resistor layer of each evaluation sample was calculated by the following formula (y), using the above-obtained thickness, mass and density values.

p=[(m ₁ −m ₀)/{(t ₁ −t ₀)×1 cm² ×d}]×100  Formula (y)

where p is the porosity (%) of the PTC resistor layer; m₁ is the mass (g) of the laminate A; m₀ is the mass (g) of the aluminum foil; t₁ is the thickness (cm) of the laminate A; t₀ is the thickness (cm) of the aluminum foil; and d is the density (g/cm³) of the PTC resistor layer.

(2) Measurement of the Electronic Resistance

As shown in FIG. 3, a tester (“40” in FIG. 3, product name: RM3545, manufactured by: Hioki E.E. Corporation) was connected to the evaluation sample 50, thereby producing the circuit 200 for electronic resistance measurement. The electronic resistance of the evaluation sample 50 in a room temperature (25° C.) condition was measured by use of the circuit 200 for electronic resistance measurement.

The following Table 1 is a table comparing the roll pressing pressures, porosities and electronic resistances of the evaluation samples of Examples 1 to 3 and Comparative Example 1.

TABLE 1 Roll pressing Electronic pressure Porosity resistance (kN/cm) (%) (%) Example 1 5.6 13 54 Example 2 14.2 5 29 Example 3 22.4 5 33 Comparative — 33 100 Example 1

3. Conclusion

According to Table 1, the porosity of Comparative Example 1 is 33% and high. This result means that for the evaluation sample of Comparative Example 1, the inside of the PTC resistor layer is relatively sparse and, as a result, the number of electron-conductive paths inside the PTC resistor layer is small.

The porosities of Examples 1 to 3 are from 5% to 13% each. These results means that for the evaluation samples of Examples 1 to 3, the inside of the PTC resistor layer is relatively dense and, as a result, many electron-conductive paths are contained inside the PTC resistor layer.

According to Table 1, each of the electronic resistance values of Examples 1 to 3 in a room temperature condition, is 29% to 54% of the electronic resistance value of Comparative Example 1 in a room temperature condition.

Accordingly, since the porosity of the PTC resistor layer is in the specific value range, more electron-conductive paths than ever before are contained inside the PTC resistor layer, and excellent electron conductivity inside the PTC resistor layer is obtained. As a result, it was proved that when the electrode is used in a solid-state battery, an increase in the electronic resistance inside the PTC resistor layer is suppressed, and a decrease in the performance of the solid-state battery is suppressed.

As described above, the results of the electronic resistance measurement of the evaluation samples can be said to be test results reflecting the performance of the solid-state battery itself (FIG. 4).

REFERENCE SIGNS LIST

-   1. PTC resistor layer -   2. Electrode active material layer -   3, 3′. Current collector -   10. Electrode for solid-state batteries -   20. Electrolyte layer -   30. Opposite electrode -   40. Tester -   50. Evaluation sample -   100. Solid-state battery -   200. Circuit for electronic resistance measurement 

1. An electrode for solid-state batteries, wherein the electrode comprises an electrode active material layer, a current collector and a PTC resistor layer disposed between the electrode active material layer and the current collector; wherein the PTC resistor layer contains an electroconductive material, an insulating inorganic substance and a polymer; and wherein a porosity of the PTC resistor layer is from 5% to 13%.
 2. The electrode for solid-state batteries according to claim 1, wherein the insulating inorganic substance is a metal oxide.
 3. The electrode for solid-state batteries according to claim 1, wherein the electroconductive material is carbon black.
 4. A solid-state battery comprising a cathode, an anode and an electrolyte layer disposed between the cathode and the anode, wherein at least one of the cathode and the anode is the electrode for solid-state batteries defined by claim
 1. 